Enhanced uplink (EUL) is proposed in the Third Generation Partnership Project (3GPP) Release 6 to improve uplink performance of Wideband Code Division Multiple Access (WCDMA) systems. Two transmission time intervals (TTIs) are proposed for enhanced uplink, a ten (10) milliseconds TTI and a two (2) milliseconds TTI. The ten milliseconds TTI provides similar cell coverage as previous Universal Mobile Telecommunications System (UMTS) releases, but its cell throughput is too small. The two milliseconds TTI provides better cell throughput than the ten milliseconds TTI, but its cell coverage is insufficient.
One technique that improves enhanced uplink coverage is hybrid automatic repeat request (HARQ) retransmission. According to current 3GPP specifications, enhanced uplink HARQ retransmission can occur only after expiration of a round trip time (RTT). Since the RTT for the two milliseconds TTI is sixteen (16) milliseconds and three (3) retransmissions may be required to guarantee reliable reception of a packet, a retransmission delay of forty-eight (48) milliseconds could be introduced by enhanced uplink HARQ. Such a delay may not be acceptable for some delay sensitive services (e.g., voice over Internet protocol (VoIP) services, etc.).
Autonomous retransmission has been proposed as an effective way to reduce such HARQ retransmission delay. A core concept of autonomous retransmission is that user equipment (UE) sends a number of retransmissions consecutively, without waiting for receipt of a negative acknowledgment (NACK) before starting the next retransmission. However, if three retransmissions are required, autonomous retransmission may only reduce the retransmission delay to six (6) milliseconds. Some autonomous retransmission techniques describe how a receiver knows that a bundle of transmissions are designated for a single packet, how the receiver decodes the packets transmitted with autonomous retransmission correctly, how to apply autonomous retransmission in high-speed downlink packet access (HSDPA), how to use autonomous retransmission for extended coverage, how to apply autonomous retransmission to notify a non-serving Node B, etc.
Although autonomous retransmission is an effective way to improve uplink coverage, it still suffers from several drawbacks. For example, if autonomous retransmission is applied with an excessively large number of transmission attempts or at an inappropriate time, autonomous retransmission will generate unnecessary interference in a system. On the other hand, autonomous retransmission with less (or an inadequate number of) transmission attempts can not take full advantage of the benefits of the technique.